Conventionally, an extraction steam turbine utilized for power generation plant having an extraction pipe for extracting some of steam from the steam turbine driving a generator and supplying the extracted steam to a production process using the steam has been generally known, for example, as described in Patent Document 1. The configuration of this steam turbine power generation plant described in Patent Document 1 is shown in FIG. 12.
In FIG. 12, reference numeral 1 represents a boiler, reference numeral 2 represents an extraction steam turbine provided with a high-pressure portion 3 and a low-pressure portion 4 each of which has blade rows, reference numeral 5 represents a condenser for condensing steam discharged from the extraction steam turbine 2, reference numeral 6 represents a generator directly coupled to and driven by the turbine 2, and reference numeral 7 represents a deaerator for heating and deaerating condensate water obtained from the condenser 5. Note that the extraction steam turbine 2 is also provided with a main steam control valve 8 for controlling the flow rate of main steam flowing into the high-pressure part 3, and extraction control valves 9 for controlling the flow rate of steam flowing from the high-pressure part 3 into the low-pressure part 4 to control the pressure of extraction steam.
A feedwater supply system 10 connected to the condenser 5 and the boiler 1 has a condensate pump 11, a first low-pressure feedwater heater 12, a second low-pressure feedwater heater 13, the deaerator 7, a feedwater pump 14, and a high-pressure feedwater heater 15. An extraction pipe 19 for supplying a process 16 with extraction steam, which is controlled to have a predetermined pressure by the extraction control valves 9, is connected to an outlet of the high-pressure part 3 of the extraction steam turbine 2. Note that reference numeral 20 represents a main steam pipe that is connected to the boiler 1 and the main steam regulating valve 8.
An extraction pipe for high-pressure feedwater heater 21 branches off from the extraction pipe 19 in order to be connected to the high-pressure feedwater heater 15, and has a check valve 22 and a stop valve 23. Note that reference numeral 24 represents a drain pipe that leads drain of the high-pressure feedwater heater 15 to the deaerator 7. An extraction pipe for deaerator 25 is connected to the low-pressure part 4 of the extraction steam turbine 2 and to the deaerator 7, and has a check valve 26 and a stop valve 27.
An extraction pipe for second low-pressure feedwater heater 29 is connected to the low-pressure portion 4 of the extraction steam turbine 2 and to the second low-pressure feedwater heater 13, and has a check valve 30 and a stop valve 31. An extraction pipe for first low-pressure feedwater heater 33 is connected to the low-pressure portion 4 of the extraction steam turbine 2 and to the first low-pressure feedwater heater 12, and has a check valve 34 and a stop valve 35. Note that reference numeral 36 represents a drain pipe that leads a drain of the second low-pressure feedwater heater 13 to the first low-pressure feedwater heater 12.
In such a configuration, main steam supplied from the boiler 1 has its flow rate controlled by the main steam control valve 8, enters the extraction steam turbine 2 to flow through high-pressure blade rows of the high-pressure part 3 and low-pressure blade rows of the low-pressure part 4, and thereby rotates a turbine rotor to perform a task. Also, steam discharged from the low-pressure part 4 flows into the condenser 5 that is kept at a pressure lower than atmospheric pressure, and is then formed into condensed water. It should be noted that thrust force that acts on the turbine rotor due to the steam flowing through the casings of the high-pressure part 3 and the low-pressure part 4 is supported by a thrust bearing. Extraction steam that is obtained from steam discharged from the outlet of the high-pressure part 3 is controlled to have a predetermined pressure by the extraction control valves 9 and then supplied to the process 16 via the extraction pipe 19.
In the configuration shown in FIG. 12, some of the extraction steam is supplied to the high-pressure feedwater heater 15 via the extraction pipe for high-pressure feedwater heater 21 that branches off from the extraction pipe 19.
The rest of the steam that is extracted from the extraction pipe 19 of the high-pressure part 3 is discharged and supplied to the low-pressure part 4 through the extraction control valves 9.
The generator 6 generates electric power that corresponds to the work of the task that the steam performs by flowing through the high-pressure portion 3 and the low-pressure portion of the extraction steam turbine 2 and rotating the turbine rotor.
The condensate water in the condenser 5 is pressurized by the condensate water pump 11 of a feedwater supply system 10 and fed to the first and second low-pressure feedwater heaters 12 and 13. In these feedwater heaters 12, 13, the condensate water is heated by uncontrolled extraction steam that flows from the low-pressure part 4 through the extraction pipes for first and second low-pressure feedwater heaters 33, 29. The heated condensate water flows into the deaerator 7 and is then heated and deaerated by uncontrolled extraction steam that is supplied from the low-pressure part 4 through the extraction pipe for deaerator 25.
The condensate water that is deaerated by the deaerator 7, which is the feed water, has its pressure increased by the feedwater pump 14, flows into the high-pressure feedwater heater 15, is then heated at this high-pressure feedwater heater 15 by uncontrolled extraction steam that flows from the high-pressure part 3 through the extraction pipe for high-pressure feedwater heater 21, and then supplied to the boiler 1. The feedwater supplied to the boiler 1 is heated into steam and then supplied to the extraction steam turbine 2 as the main steam.
In this type of general extraction steam turbine power generation plant, the steam coming from the boiler flows through the extraction steam turbine 2, and thereby rotates the turbine rotor to perform a task. Thereafter, the steam becomes condensate water at the steam condenser, which is supplied to the boiler 1 as feedwater and circulates among the boiler 1, the extraction steam turbine 2, and the condenser 5. In this circulation, the steam extracted from the extraction steam turbine 2 has its pressure controlled to be predetermined pressure by the extraction control valves 9 and is then supplied to the process 16 and to the feedwater heaters 12, 13, 15 and the deaerator 7 in an uncontrolled manner via the extraction pipes 33, 29, 21, 25, which have check valves and stop valves or only stop valves, thereby heating the feedwater.
As the extraction steam turbines which supply extraction steam to the process 16 are designed to be operated at their most efficient operating design point for usual operating conditions in which the steam turbines are kept operated for the most of their service life, so, for example, if a ratio of extraction steam flow to main steam flow at the design point should be very large and also the ratio at current operating point should be going to be smaller than that at the design point due to drastic decrease in demand for extraction steam, then the extraction control valves will be opened wider, resulting in drastic increase in the flow rate of steam flowing in the low-pressure casing 4 through blade rows of the extraction steam turbines in comparison with that at the nominal, usual operating point. As a result of this increase in the steam flow rate, the force received by the blades of the blade rows in the low-pressure part 4 increases, hence the stress added to the blades and thrust force acting on the turbine rotor. Thus, even when the stress acting on the blade stages is equal to or lower than a permissible value, excessive thrust force acts on the turbine rotor, resulting in possible damage to the thrust bearing.
In the extraction steam turbine 2 in which the extraction control valves 9 control the flow rate of the extraction steam, the steam, which flows through the blade rows of the low-pressure part 4 downstream of the extraction control valves 9, does not flow in an amount that cannot be tolerated by the fully open extraction control valves 9. However, in the system in which the uncontrolled extraction steam is supplied from the low-pressure part 4 to the plurality of feedwater heaters and the like, when, for example, the extraction pipe for deaerator 25 and the check valves 26 and 30 of the extraction pipe for second low-pressure feedwater heater 29 shown in FIG. 12 are damaged due to chatter caused by decrease in steam flow rate or vibration caused by excessively high steam flow velocity, and consequently the supply of the extraction steam to the feedwater heaters is stopped, the flow rate of the steam flowing to the low-pressure part 4 becomes greater than a defined amount. This involves a risk of excessive thrust force acting on the blade rows of the low-pressure part 4 or damage to the thrust bearing, as described above.
Patent Document 1 discloses a safety operation apparatus for extracting some of the steam discharged from the high-pressure part 3 with blade rows, supplying the extracted steam to the process and the like, and preventing damage to the thrust bearing from excessive thrust force that is caused by an increase in the flow rate of the rest of the steam flowing through the low-pressure part 4 with blade stages.
The safety operation apparatus disclosed in Patent Document 1 is shown in FIG. 13.
In the extraction steam turbine 2 that has the high-pressure part 3 and the low-pressure part 4, each of which has blade rows, the main steam control valve 8 controls the flow rate of steam flowing into the high-pressure part 3, and the extraction control valves 9 control the flow rate of steam flowing from the high-pressure part 3 into the low-pressure part 4, thereby controlling the pressure of steam that is extracted from the discharged steam of the high-pressure part 3 and supplied to the process 16. The extraction steam turbine 2 has an arithmetic control device 42 that provides opening commands OP8 and OP9 to a main steam control valve controller 44 for controlling the main steam control valve 8 and an extraction control valve controller 46 for controlling the extraction control valve 9, in response to detection signals from a high-pressure casing pressure detector 40 that detects the pressure of the steam of the high-pressure casing of the high-pressure part 3, and a low-pressure casing pressure detector 41 that detects the pressure of the steam of the low-pressure casing of the low-pressure part 4.
The arithmetic control device 42 functions to adjust and compute the pressure of the extraction steam. The arithmetic control device 42 compares steam pressure (“extraction pressure,” hereinafter) Pp of the extraction pipe 19, which is detected by a extraction pressure detector 48 installed in the extraction pipe 19 coupled to the process 16, with set pressure Pps set by an extraction pressure setter 49, and creates the valve opening signal OP9 so that the extraction pressure Pp is equal to the set pressure Pps. A valve operation signal based on this valve opening signal OP9 is output from the extraction control valve controller 46, and converted into hydraulic signal by an electric-hydraulic converter 47, which is then provided to the extraction control valves 9. In this manner, the pressure of the extraction pipe 19 connected to the extraction steam turbine 2 and the process 16 to each other is controlled constantly to the set pressure by the arithmetic control device 42 and the extraction control valve controller 46. As a result, the pressure in the process 16 is kept at constant value.
During load (generating) operation of the extraction steam turbine 2, the arithmetic control device 42 compares low-pressure casing detection pressure Lp with low-pressure casing reference pressure Lpp that is delivered unambiguously by the high-pressure casing pressure Hp as a value on a correlation line P expressed as a linear equation formed by a special relation between the high-pressure casing pressure Hp and the low-pressure casing pressure Lp of the extraction steam turbine 2 as shown in FIG. 2. The special relation is a relationship between the Hp and the Lp, in which the thrust force generated in the extraction steam turbine 2 becomes a certain constant value depending on the combination of the flow rate of steam passing through the high-pressure part 3 and the flow rate of steam passing through the low-pressure part 4.
In other words, the correlation line P shown in FIG. 2 expresses a relationship between the flow rate of steam passing through the high-pressure part 3 and the flow rate of steam passing through the low-pressure part 4 when the value of the thrust force is a certain constant value during extraction pressure control operation of the extraction steam turbine 2. When the low-pressure casing pressure Lp is higher than the low-pressure casing reference pressure Lpp delivered in relation to the high-pressure casing pressure, and falls within the range of oblique lines above the correlation line P in FIG. 2, the arithmetic control device 42 sends an automatic control cancellation command CS to the extraction control valve controller 46 to cancel the extraction pressure control, and outputs the valve opening command OP8 to the main steam control valve 8 to reduce the valve opening thereof so that the low-pressure casing detection pressure Lp becomes lower than the low-pressure casing reference pressure Lpp delivered in relation g to the high-pressure casing pressure according to the correlation pressure relationship and that the load of the extraction steam turbine 2 becomes lower than the current load.
Note that the correlation line P in this case can express a relation between the low-pressure casing pressure and the high-pressure casing pressure that are obtained when the thrust force is equal to a value equivalent to a tolerance surface pressure of the thrust bearing.
According to such a safety operation apparatus, when the casing pressure Lp of the low-pressure portion 4 in the extraction steam turbine 2 falls within the range of oblique lines above the correlation line P of FIG. 2, the arithmetic control device 42 controls the main steam control valve 8 and the extraction control valves 9 to reduce the casing pressure Lp of the low-pressure portion 4 to a pressure below the correlation line P of FIG. 2 and operate the turbine. The safety operation apparatus, therefore, can prevent damage to the thrust bearing of the extraction steam turbine without causing excessive thrusting force to act on the thrust bearing.
Patent Document 1: Japanese Patent No. 3186468